1. Field of the Invention
The present invention relates to a compression air-conditioning system for a railroad car, intended in particular for a high-speed train.
2. Description of Related Art
In general, a compression air-conditioning system of this type chiefly comprises:
a compressor into which a refrigerant is admitted, compressed and expelled under pressure into a downstream circuit;
a condenser, connected to the outlet of said compressor and in which the refrigerant is cooled by the air issuing from motor fans, which causes it to liquefy;
an expansion valve followed by an evaporator, in which the refrigerant, having been subjected to a relatively very low pressure, vaporizes, which causes its temperature to be lowered and the desired cooling to be produced;
a circuit returning the gaseous refrigerant to the inlet of the compressor after it has passed through an air-treatment unit in which it is reloaded with heat.
The air-conditioning system is usually situated beneath the floor of the car, in a limited space between this floor, a wall parallel to this floor and lateral and transverse walls, said lateral walls being provided with openings for the inlet and outlet of the cooling air.
The condenser in such a system functions under a relatively high pressure corresponding to the condensation pressure of the refrigerant, whereas the evaporator functions under a relatively low pressure corresponding to the evaporation pressure of the fluid. The high-pressure circuit is connected to the low-pressure circuit, on the one hand, by the compressor and, on the other hand, by the expansion valve.
Various disadvantages have been noted in trains equipped with such systems:
in certain atmospheric conditions (snow, frost, etc.), it may be observed that the blades of the motor fans ice up, which can cause the entire system to come to a standstill, and yet the surrounding cold could enable the system to function with a reduced number of motor fans, or even with no fans at all, without risking damage to the equipment;
the increase of the heat loads, during a long period of sunshine or with a very high number of occupants, does not enable the desired comfort in terms of temperature to be ensured, given the limitation of the space available for the air-conditioning system;
the compressor-condenser unit generates vibrations which are a nuisance, in particular for the comfort of the passengers. This disadvantage is further amplified with the increase in the speed of the trains, which is desired by rail passenger carries, and in view of which the moving masses are reduced, as this reduction enables higher speeds to be attained for an equivalent moving force. Lightening the structures, however, makes them more sensitive to the vibrations which are felt by the passenger magnified at the level of the floor and the seat, which reduces the level of comfort.
In this type of system, a reduction of the vibrations is made difficult by the fact that a refrigerating compressor is necessarily connected to the high-pressure circuit by a rigid pipework which is required by the nature of the (refrigerant) fluid conveyed and by the high pressure. The result of this in practice is that the vibrations of the moving mechanical element which forms the compressor are transmitted to the whole refrigerating circuit.
Furthermore, the strict limitation on the space available for the system makes it impossible for the conventional shock-absorbing systems to be fitted to it.
In this respect, it should be pointed out that, in existing cars where the system is located in the lower part of the car, immediately below the body, the tubes of the condenser in which the refrigerant liquefies are arranged horizontally between, on the one hand, the floor of the body and, on the other hand, a plane wall parallel to this floor and integral with the latter. At their ends, the tubes are fixed to two vertical parallel plates in which these ends are regularly distributed. Furthermore, horizontal plates forming fins are arranged at regular intervals between the tubes so as to channel the air flow produced by one or more horizontal-axis fans. The assembly of the tubes of the condenser thus forms one or more banks of tubes, these banks being traversed by a flow of cooling air.
However, because of the limitation on the available space, in particular in the vertical direction relative to the floor of the car, assumed to be horizontal, the cooling capacity of the system is itself limited, which may have an adverse effect on the comfort of the passengers.